Process for the production of ion-emitting surfaces, particularly for halogen leak detectors



Aplll 11, MOESTA PROCESS FOR THE PRODUCTION OF ION-EMITTING SURFACES.PRTICULARLY FOR HALOGEN LEAK DETECTORS Filed May 14, 1958 INVENTOR. Hasso M0 csfa.

United States Patent() Hasso Moesta, Koln-Weidenpesch, Germany, assignorto y National Research Corporation, Cambridge, Mass., a corporation ofMassachusetts Filed May 14, 1958, Ser. No. 735,327

1 Claim. (Cl. 313-63) Ion sources are known in which suitable materials,chiefly alkali or alkaline earth metals and/or their compounds, are usedas ion-forming activated materials, which are suspended in a suitablyshaped storage chamber. Such a chamber, preferably a ceramic tube, isheated by a special heating device,perhaps by a current-carrying wirecoil.

In a known method of performance the activated materials to bevolatilized are deposited on the material of a small ceramic tube. Thetube is then surrounded with aheating coil, which causes the evaporationof the volatile substances. At a slight distance from this evaporationset-up formed from the small ceramic tube and the heating coil belongingto it there is another coil which serves as the anode.r

In another known construction of an ion source alkali or alkaline earthmetals are evaporated from a storage chamber further removed from theanode, so that the atoms of the metal vapor formed at the aperture areionized by the perforated or porous anode surface.

Special problems arise with the use of ion sources in so-called halogenleak detectors as they are used in chemical apparatus for the locatingof'leaks. In these instruments an ion source is necessary which isresistant to air penetration with suflicient temporary stability. Theion source must, moreover, show a high sensitivity to halogens so thatvery small traces provoke at once a marked change of ion emission. Onoperating such a halogen leak detector ions are released by the ionsource and are absorbed by a preferably cold cathode. If halogen vapor(for example CFzCl-z) is formed around such an ion source, the stream ofions is altered and an attached meter indicates the presence of thehalide. For detection a vacuum must iirst be produced in the apparatus'and after this the outer surfaces of the seal are swept for possibleleak points with a halogen containing test gas, for example CF2C12.Under the effect of the surrounding air test gas diffuses through theunsealed points linto the inside of the apparatus and is passed on intothe attached leak detector tube with the ion source. There the iniiuenceof the halogen vapor produces a correspondnig change of emission at theanode. In this way leaks in extensive and complicated apparatus of thelarge chemical industries can be located in a short time. t

A known method of making an ion source for halogen leak detector isshown in the sketch. In a small ceramic tube 1 which contains a suitablesupply of activated material (for example potassium compounds) isarranged a PatentedvApr. 11, 1961 in a discharge gap which the ionsarising from the yanode areabsorbed by a cold cathode 4. The method of"operating suchaion source is as follows: First the vaporized metallicatoms emerge from the surface of the small ceramic tube and diffusesubsequently, preferably along the grain boundaries of thepolycrystalline metal casing of the covering 3, to its outer surfacewhere in the charging field ions of the activated substance are formed.

The duration and sensitivity of such an ion source depend now on thesize and the time constant of the diffusion process of the metal atomsthrough the covering 3. In the practical production of such ion sourcesgreat diiiiculties arise since the structure of the grain boundary ofthe platinum sheet varies from lot to lot and since likewise varyingporosity acts in a similar way. After the discovery that theelfectiveness of such ion sources depends on the diffusion of theactivated material along the grain boundaries and through porous places,porous sintered metal has been used for the covering 3. Such poroussintered shapes of platinum are uneconomical.

The present new invention discloses a process for the production ofion-emitting surfaces, especially for halogen leak detectors, in whichthe activated substance, vaporizing from a supply source, passes througha plate-shaped anode and comprises depositing foreign atoms in measuredamount on the crystalline getterer of the anode sheets and in thecrystalline interstices. By this preparative process the structure ofthe grain boundaries of the anode plates are so aiected that practicallywith no waste there can be accomplished the production of ion sourceswhich show a high and uniform emission of ions and marked sensitivity ofthe electrode toward halogens. The foreign atoms which are to beembedded must be so supplied that an expansion of the getteringstructure and of the ntergranular space occurs. They are furthermore soselected that the process of causing the appropriate sensitivity at theanode is not lessened. With an anode plate of platinum the deposit ofcarbon as the foreign atoms has worked well.

In a particularly favorable embodiment of the new process the carbonatoms are embedded in the platinum Y sheet by thermal diffusion. Forthis purpose the platinum sheet, designed as a tube, is mounted on asuitable carbon arc lamp, and it is embedded for a certain period atelevated temperature, preferably in a reducing atmosphere. Temperaturesbetween about 500 C. and 1000 C. preferably about 700 C. can be suitablyemployed. In place of deposition in the reducing atmosphere, embeddingcan be done in a vacuum. According to the duration of the annealing,annealing temperature and raw material, one obtains different high ion'currents from the ion source produced and different high diffusioncoefficients of the activated material through the anode metal. Theseparameters can be varied within broad limits and thus it is possible toproduce ion sources which are adjusted to the most differentrequirements with respect to duration, yield, and sensitivity. fy

What is claimed is:

A detector for a linely divided atmospheric substance capable ofinducing the formation of positive ions at a conductive surfacecomprising an electrical discharge device including a pair ofelectrodes, means for causing a quantity of an atmosphere containingsaid substance to pass between said electrodes, means for imposing anelectric potential difference between said electrodes, and means fordetecting changes in the current between said electrodes caused by theformation of positive ions at one of said electrodes due to the presenceof said substance, the electrode at which said ions are formedcomprising a platinum sheet-like anode having adjacent thereto a ceramiccontainer for activated material, said anode having carbon embeddedtherein so that said activated maten'al may penetrate said anode throughinterstices present in said platinum carbon material.

References Cited in the tile of this patent UNITEDfsTATE-s PATENTSyYllilliams Feb. 14, 1950 Rice Apr. 24, 1951 Morgan Sept. 29, 1953 DeJuren Aug. 16, 1955 Zemany Apr. 17, 1956 Bouty July 10, 1956 Bouty May20, 1958

